Experiments 1-3 are designed to determine why diffuse and Localized Pavlovian S plus cues sometimes facilitate, and sometimes suppress operant responding. From a peripheral viewpoint, it is believed that facilitation is due to additive summation of compatible behaviors, whereas suppression is due to subtractive summation of incompatible behaviors. From a central viewpoint, behaviors summate primarily when the baseline operant is under only response-reinforcer control, whereas compatibility and incompatibility of behaviors is overidden by associative summation when the operant is under both response-reinforcer and stimulus-reinforcer control. Experiments 1 and 2 will test these accounts by using diffuse S plus cues with rats in two entirely different experimental procedures. Experiment 3 will use one of these procedures to extend these accounts to localize S plus cues with pigeons. Experiment 4 will generalize the rationale behind Experiments 1-3 to a situation in which negative response-reinforcer and positive stimulus-reinforcer contingencies exist within the same component of a two-component multiple schedule. Prior evidence, together with the implications of the peripheral and central accounts, suggest that the negative response-reinforcer contingency should affect only the response upon which this contingency was established, whereas the positive stimulus-reinforcer contingency should affect a response upon which this contingency was not established.